Cathode ray tube

ABSTRACT

In a cathode ray tube of the type in which a plurality of electron beams are directed toward a phosphor screen by a beam generating assembly and are made to intersect each other substantially at the optical center of a main focusing electron lens by which all of the beams are focused on the screen, the usual grid electrode assembly is provided with an additional or auxiliary electrode for at least one of the beams, which auxiliary electrode has applied thereto a selected voltage by which the beam cutoff voltages for the plural beams can be maintained at selected values while permitting independent control of the modulation conditions for the several beams.

United States Patent Miyaoka [4 1 July 18, 1972 [54] CATHODE RAY TUBE2,862,144 11/1958 3,028,52l 4/ I962 [72] lnventor. Senrl lvllyaolia,Kanagawa ken, Japan 2,888,606 5/!959 [73] Assignee: Sony Corporation,Tokyo, Japan 2,935,642 5/1960 [22] Filed: June 22, 1970 PrimaryExaminer-Benjamin A. Borchelt [211 App! 484181 Assistant Examiner-S. C.Buczinski Aimrney- Lewis H. Eslinger. Alvin Sinderbrahd Curtis. 30Foreign Application Priority pm Morris & Safford June 30, 1969 Japan,..44/51s93 June 30, I969 Japan ..44/s I892 [571 ABSTRACT In a cathoderay tube of the type in which a plurality of elec- [52] U.S.Cl...3l5/l3R,3l5/l3 CG, 3311553801, "on beams are directedtowafdaphosphorScreen byabeam [51 I Int. Cl H01 29/50 generatingassembly and are made to intersect each other sub 58] Field Search315/13 R 13 C 13 3 stantially at the optical center of a main focusingelectron lens 5 by which all of the beams are focused on the screen, theusual grid electrode assembly is provided with an additional orauxiliary electrode for at least one of the beams, which auxiliary [56]References Cited electrode has applied thereto a selected voltage bywhich the UNITED STATES PATENTS beam cutoff voltagesfor the plural beamscan be maintained at selected values while permitting independentcontrol of the 2'2 1 1;; i 5 23 1 modulation conditions for the severalbeams.

0s 1 a e 3,289,034 1 1/1966 Weber et a]. ..3l3/8l 14 Claims, 11 DrawingFigures PATENTEU JUL] 8 I972 w h nmnlillll K $2 M x k o m mwh Y Q I M Q.A 1 w. M X i u% R H Fla M E \m w I M n a. 6 7 MN qwwim W G F n n u I I IPATENTEU JUL] 81912 SHEET 0F 4 CATHODE RAY TUBE This invention relatesgenerally to cathode ray tubes of the single-gun. plural-beam type, andmore particularly to improvements in cathode ray tubes of that type inwhich the plurality of electron beams are passed substantially throughthe optical center of a common electron lens by which all of the beamsare focused on the color phosphor screen.

In single-gun, plural-beam cathode ray tubes of the type to which thisinvention relates, for example, as disclosed in detail in the U.S. Pat.No. 3,448,316, issued June 3, 1969, and having a common assigneeherewith, a plurality of electron beams are originated by a beamgenerating cathode assembly comprising, for example, a plurality ofcathodes, and are converged to intersect each other substantially at theoptical center of a single main electron lens by which the beams arefocused on the color screen, thereby to diminish optical aberrationsimparted to the beams in the course of the focusing thereof by the mainelectron lens.

Converging of the beams to intersect each other substantially at theoptical center of the main electron lens and simultaneous pre-focusingof the beams are effected by an auxiliary electron lens or the likewhich is common to all of the beams and located between the cathodeassembly and the main electron focusing lens. When the beams areconverged to intersect each other substantially at the optical center ofthe main electron lens, at least certain of the beams emerge from thelens along divergent paths, and pairs of convergence deflecting platesmay be arranged along such divergent paths and have voltages appliedtherebetween to deflect the beams in directions for causing all of thebeams to converge at a common point on an apertured beam selecting grillor mask placed adjacent the color screen so as to produce color pictureson the screen. Further, the beams are simultaneously deflected by themagnetic fields resulting from the horizontal and vertical sweep signalsapplied to corresponding coils of a deflection yoke to cause the beamsto scan the screen.

In such single-gun, plural-beam cathode ray tubes in which the electronbeams are simultaneously emitted from the cathodes, dispersion isintroduced in the modulation characteristics of the beams by voltagevariations in the video input signals applied to the cathodes or grids,or therebetween, as is the case with usual plural-gun cathode ray tubesemploying three electron guns, and it is necessary to make themodulation characteristics of the beams uniform so as to providepictures with good quality. To this end, it has been preferred that asecond grid of the gun be divided into individual second grids eachcorresponding to a respective cathode of the electron gun emitting acorresponding electron beam. However, in the electron gun of the type inwhich the plural beams are prefocused by a common auxiliary electronlens, the second grid plays an important role in forming a potentialdistribution which is symmetrical with respect to the axis of the gunand which constitutes the common auxiliary electron lens. Accordingly,from the viewpoint of avoidance of distortion of the electric field ofthe auxiliary electron lens, it is undesirable to divide the second gridinto individual second grids to which different voltages are applied.Further, the provision of the independent second grids for the pluralbeams introduces complexity in the construction of the electron gun.

Accordingly, it is an object of this invention to provide an improvedcathode ray tube of the aforementioned type which provides a picture ofhigh quality on the screen.

More specifically, it is an object of this invention to provide animproved cathode ray tube of the aforementioned type in which the beammodulation conditions for the several beams may be controlledindependently ofeach other.

It is still another object of this invention to provide an improvedcathode ray tube of the aforementioned type which is suitable for use inlarge size tubes.

In accordance with an aspect of this invention, a cathode ray tube ofthesingle-gun, plural-beam type is provided with a grid electrode assemblyhaving auxiliary electrodes to which desired potentials are applied.Since the beam cutoff voltage and beam modulating condition aredependent upon the potential difference between the first grid electrodeand the auxiliary electrode, they can be changed by controlling thepotential applied to the auxiliary electrode. Accordingly, the beamcutoff voltages for the respective beams can be adjusted to a commondesired value by supplying the auxiliary electrodes with selectedpotentials and the beam modulation condition for each beam may becontrolled independently of the others without causing distortion in theelectron lens by which the beams are converged and/or prefocused.

The above, and other objects, features and advantages of this invention,will become apparent from the following description of illustrativeembodiments which is to be read in conjunction with the accompanyingdrawings, wherein:

FIG. I is a schematic view showing a prior art single-gun plural-beamcathode ray tube of the type to which this invention may be applied;

FIG. 2 is a schematic cross-sectional view illustrating the principalpart of a plural-beam electron gun and showing one embodiment of thisinvention incorporated therein;

FIGS. 3, 4 and 5 are views similar to that of FIG. 2, but showing otherembodiments of the invention;

FIGS. 6A and 6B and FIGS. 7A and 7B are schematic detail views to whichreference will be made in explaining the invention; and

FIGS. 8 and 9 are schematic views similar to that of FIG. 2, but showingfurther modified forms of this invention.

For a better understanding of this invention, a description will begiven of the type of plural-beam electron gun to which this invention isapplied, and which is shown in FIG. 1. The reference character Aindicates generally the electron gun, in which three cathodes K K and Kare aligned, for example, in a horizontal direction. The gun A furtherincludes a first cup-shaped common grid 0,, a second cup-shaped commongrid G and third, fourth and fifth tubular grids G G,'and G sequentiallyarranged coaxial with the center cathode K,;. The first grid G, hasapertures g g and g formed therein in alignment with-the cathodes K Kand K and the second grid G has apertures g g and g formed therein inalignment with those of the first grid 0,. A voltage of 0-400 V. isapplied to the first grid 0,, a voltage ofO to 500 V. is applied to thesecond grid 0,, a voltage of 13 to 20 KV. is applied to the third andfifth grids G and G and a voltage of0 to 400 V, is applied to the fourthgrid 0,. With such voltage distribution, an auxiliary electron lens L isformed between the second and third grids G and G and a main electronlens L is formed by the third, fourth and fifth grids G G, and Gapproximately at the axial center of the fourth grid G The auxiliarylens L prefocuses the three beams B 8,; and B emitted from the cathodesK,,, K and K and causes the side beams B and B to converge so that theycross or intersect with'the center beam B substantially at the opticalcenter of the main lens L and diverge therefrom. An electrostaticconvergence deflecting means C is provided to deflect the side beams Band B The convergence deflecting means C comprises, for example, a pairof opposed shielding plates P and P permitting the passage therebetweenof the center beam 8,; and deflector plates Q and Q respectivelydisposed in opposed relationship to the shielding plates P and P forconvergently deflecting the side beams 8,, toward the center beam B Theshielding plates P and P are supplied with, for example, an anodevoltage V of the cathode ray tube which may be the same as the voltageof the fifth grid G so that the center beam 8 passes undeflected betweenthe plates P and P and impinges on a color screen S. The deflectorplates Q and Q are supplied with a convergence voltage V which is lowerthan the anode voltage V by about 200 to 300 V., so that the beams 8,,and B,, passing between the plates Q and Q are deflected in the mannerof an optical prism to be converged on the screen S together with thebeam B,,-. In this manner, the three beams B 8,; and B passing throughthe convergence deflecting means C impinge on the screen S. The screen Sis made up of, for example, red, green and blue phosphor strips 5 8,,-and 5,,

sequentially arranged, for instance, vertically in a repeating cyclicorder. Adjacent the screen S there is provided a beam selecting grid 6,.consisting of grids or slits g, each placed in front of a correspondingarray or triplet of the red, green and blue phosphor stripsS S and 8,.The beam selecting electrode G, is supplied with a high voltage V,., bywhich the three beams 8,, B and B are directed between adjacent grids atpredetermined incidence angles to land on the red, green and bluephosphor strips 8,, S and 8,. Reference character D designates theelectromagnetic deflecting means for deflecting the beams horizontallyand vertically so as to scan the screen.

Referring now to FIG. 2, which shows one example of the plural-beamelectron gun with an embodiment of this invention incorporated therein,it will be seen that those elements corresponding to the gun of F IG. 1are identified by the same reference characters and will not be furtherdescribed.

In accordance with the present invention, auxiliary electrodes G,,,,G,,; and G5,, for adjusting cathode cutoff voltages are interposedbetween the first and second grids G, and G, to respectively influencethe beams emitted from the cathodes K,,, K, and K The auxiliaryelectrodes G,,,, 6' and G,,, are supplied with positive potentials V,,V, and V, relative to the cathodes K,,, K,- and K,,, have formed thereinapertures g,,,, g' and g,,, in alignment with the cathodes K K and K,,

With such an arrangement, the cutoff voltages of the beams 8,, B and Bare dependent upon the potential relationship among the cathodes K K andK the first grid G, and the auxiliary electrodes G, G', and G',,,.Accordingly, by suitably selecting the potentials V,, V, and V, of theauxiliary electrodes G,,,, G',,; and G,,,, the beam cutoff voltages canbe made equal to one another without asymmetrical distortion of theelectric field of the auxiliary lens L formed by the second grid 0,. Atthe same time, the bias voltage levels of the cathodes K K and K, can bemade equal to one another, thereby to provide for uniform modulationcharacteristics of the respective beams. I

The shape of the second grid G, greatly affects the formation of theauxiliary lens without distortion but its potential Vg, has littleinfluence on the prefocusing of the beams, so that at least one of theauxiliary electrodes, for example, the auxiliary electrode G, may be atthe same potential as the second grid 6,. Thus, it is possible toconnect the central auxiliary electrode G',,; with the second grid G, inadvance so as to eliminate the need for external terminals connectedwith such auxiliary electrode.

In the event that the second grid G, and, for example, the

central auxiliary electrode G,, are maintained at the same potential, itis possible to omit the central auxiliary electrode and to shape thesecond grid G, to perform the function of the omitted auxiliaryelectrode for the central beam B as shown in FIG. 3. In this case, thatportion of the second grid G, facing the central cathode K, projectsoutwardly, as at G,,,, and the projecting end face is substantiallyflush with the auxiliary electrodes and 0', disposed on both sides ofthe projection.

With such arrangement, the side beam cutoff voltages are respectivelydependent upon the potential relationship among the cathode K,,, thefirst grid G, and the auxiliary electrode G',,, and the potentialrelationship among the cathode K,,, the first grid G, and the auxiliaryelectrodes G',,,. The central beam cutoff voltage depends upon thepotential relationship among the cathode K the fist grid G, and thesecond grid G Accordingly, by suitable selection of the potentials V,,V, and Vg, of the auxiliary electrodes G,,, and G',,, and the secondgrid 6,, the cutoff voltages of the respective beams B,,, B and B, canbe made equal to one another, thereby to render uniform the beammodulation characteristics for the three beams.

While the present invention has been described as being applied to anelectron gun in which the first grid G, is common to the cathodes K Kand K the invention is also applicable to electron guns, such as areshown in FIGS. 4 and 5, in which the first grid G, is divided intoindividual grids respectively corresponding to the cathodes K K and K,,.In FIGS. 4 and 5, the respective cathodes and first grids are angularlydisplaced from each other so as to radiate from, and be equally spacedfrom the optical center of the main electron lens.

in the examples illustrated in FIGS. 3, 4 and 5, no auxiliary electrodeis provided for the central beam B and the second grid G, is used toperform the function of the'auxiliary electrode for the central beam Bin addition to performing the usual function of the second grid for theside beams B, and 8,. However, the omission of the auxiliary electrodeneed not be limited specifically to the central beam but, instead, theauxiliary electrode for any one of the beams may be omitted.

When the plural-beam electron gun has the auxiliary electrodes forrendering the beam cutoff voltages and the beam modulationcharacteristics uniform interposed between the first grid G, and thesecond grid G, for providing an axially symmetrical potentialdistribution, and the potentials of the auxiliary electrodes and thesecond grid are different from each other, an electron lens is formedbetween the second grid and the auxiliary electrodes by the potentialrelationship therebetween which exerts an influence upon the angles ofdivergence of the beams from the auxiliary electrodes.

The potential relationship between the second grid and the auxiliaryelectrodes may influence the divergent angles of the beams in twodifferent ways according to the arrangement and construction of theauxiliary electrodes. When the auxiliary electrode G, interposed betweenthe first and second grids G, and G, is placed near the former, asdepicted in H6. 6, the potential relationship between electrode G, andgrid G, will have one influence, and when the auxiliary electrode G,interposed between the first and second grids G, and G, is locatedfurther from grid G,, the potential relationship between electrode G,and grid G, will have the opposite influence, as shown on FIG. 7.

In the case of FIG. 6, when the potential V, of the auxiliary electrodeG, is lower than the potential Vg, of the second grid G,, the beamsdiverge at-an angle 6, greater than the angle 9 at which the beamsdiverge from Vg, V,, as shown on FlG. 6A. If the divergent angle of thebeams is thus made great, a wide area of the main electron lens L isused for beam focusing which is likely to be affected by aberration.

On the other hand, when-the potential V, of the auxiliary electrode G,is higher than the potential Vg, of the second grid 0,, the beamsdiverge at an angle 0, smaller than the angle 0,, in the case of V V,,as shown on H6. 68. When the divergent angle of the beams is thus madesmall, the beams pass through only a relatively small central area ofthe main electronlens L, near the optical axis of the lens so as to befocused with minimal aberration and thereby permit the bright spotformed on the screen to be reduced in diameter. Accordingly, in the casewhere the auxiliary electrode G, is located near the first grid 6,, asshown in FIGS. 6A and 68, it is preferred that the potential V, of theauxiliary electrode G, be higher than the potential Vg, of the secondgrid G, in order to improve the focusing characteristics.

In the case of the auxiliary electrode G, being'substantially spacedfrom first grid G,, for example, approximately midway between grids G,and 0,, the potential V, of the auxiliary electrode G, being lower thanthe potential Vg, of the second grid 0,, causes the beam to diverge atan angle 0, smaller than divergence angle 0 that results when V, V3,, asillustrated in FIG. 7A.

Conversely, in the event that the potential V, of the auxiliaryelectrode G, is higher than the potential Vg, of the second grid 6,, thedivergence angle 0, is then greater than the angle 0 as shown in FIG.78. Accordingly, in the case where the auxiliary electrode G, is locatedsubstantially midway between the first and second grids G, and G, asdepicted in FIGS. 7A and 7B, it is preferred that the potential V, ofthe auxiliary electrode G, be lower than the potential Vg, of the secondgrid G, for obtaining optimum beam focusing characteristics.

mlnas nass In the case where the auxiliary electrodes 0' and G areprovided for both side beams and no auxiliary electrode is provided forthe central beam, but instead the second grid G is formed to partiallyproject toward the first grid G as depicted in FIGS. 3, 4 and 5, thereis the possibility that, when the voltages of the second grid G and theauxiliary electrodes 6' andG' are adjusted for making the respectivebeam current cutoff voltages uniform, the potential relationship betweenthe second grid G and each of the auxiliary electrodes 6' and 6' may notbe in accordance with the preferred relationships described above.

Thus, if the attainment of uniform ba'ni current cutoff voltagesrequires any one of the auxiliary electrodes G and 0' to be at apotential V greater than the potential V of the second grid G then suchauxiliary electrode is disposed near to the first grid G, for attainingoptimum focusing characteristics with respect to the related beam, forexample, as in FIG. 6B. On the other hand, if the attainment of uniformbeam current cutoff voltages requires any one of the auxiliaryelectrodes G and 6' to be at a potential less than the potential Vg ofthe second grid G then such auxiliary electrode is disposed at arelatively large distance from first grid 0,, for example, midwaybetween grids G and G for attaining optimum focusing characteristicswith respect to the related beam, as shown on FIG. 7A.

Turning now to FIGS. 8 and 9, it will be seen that such views illustratefurther examples of this invention, in which individual first grids G Gand G are respectively provided for the cathodes K K and K and auxiliaryelectrodes 6' and 6' are respectively interposed between the first gridsG and G and the second grid G In the electron gun of FIG. 8, when thepotentials V of the auxiliary electrodes 6' and G' are higher than thepotential Vg of the second grid 0,, the divergence angle of each of theside beams is made smaller than that when the potentials V, and V areequal to each other, specifically by positioning the auxiliaryelectrodes 0' and G' between the first and second grids G, and G atlocations near to first grid 0,, as previously described. in such casedistance D between the central cathode K and the first grid G is madeshorter than the corresponding distances between the side cathodes K andK, and their respective first grids G and G or the aperture g of thecentral first grid 0, is formed with a diameter greater than thediameters of apertures g and g of the side first grids G and G In thiscase, the relative decrease in the distance D,, or the relative increasein the size of aperture g serves to increase the central beam cutoffvoltage. Thus, even though the potentials of auxiliary electrodes 0' andG' are greater than the potential of second grid 0., and thisrelationship would otherwise cause the side beam cutoff voltages to begreater than the center beam cutoff voltage, relatively decreasing thedistance D,, or relatively increasing the size of aperture g can serveto equalize the beam cutoff voltages for the three beams.

On the other hand, in the electron gun of the described construction,when the potentials V of the auxiliary electrodes 6' and 6' are madelower than the potential Vg of the second grid 0,, the divergence angleof each of the side beams is made smaller than that when the potentialsV and Vg are equal to each other, by disposing the auxiliary electrodesG' and G' substantially midway between the first and second grids G and(3,, as described above, and further in that case the distance D betweenthe central cathode K and the first grid G is made longer than thecorresponding distances between the side cathodes K and K and theirrespective first grids G and G as depicted in FIG. 9, or the aperture ofthe central first grid G is made smaller than the apertures g and g ofthe side first grids G and G In this case, either the relative increasein the distance D or the relative decrease in the size of the aperture gserves to decrease the central beam cutoff voltage. Thus, even thoughthe potentials of auxiliary electrodes 0', and G' are adjusted to besmaller than the potential of second grid 0;, and this relationshipwould otherwise cause the side beam cutoff voltages to be less than thecenter beam cutoff voltage, relatively increasing the distance D,, orrelatively decreasing the size of aperture g can serve to bring down thecenter beam cutoff voltage to be equal to the adjusted side beam cutoffvoltages.

Thus, in accordance with this invention, as has been described in theforegoing, it is possible to make the respective beam cutoff voltagesequal and hence render the beam modulation characteristics uniformwithout imparting any distortion to the pre-focusing lens by theauxiliary electrodes and, at the same time, the divergenceanglesof thebeams can be held small to thereby provide for improved beam focusingcharacteristics. This invention is of particular utility when employedin large size cathode ray tubes of the Trinitron (Registered Trademark)type.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the drawings, it is to be understoodthat the invention is not limited to those precise embodiments, and thatvarious changes and modifications may be effected therein by one skilledin the art without departing from the scope or spirit ofthe invention.

What is claimed is:

1. A cathode ray tube comprising: I

A. A phosphor screen;

B. Beam producing means to form a plurality of separately modulatableelectron beams to impinge on said screen after intersecting each otherat a location in the tube between said beam producing means and saidscreen, said beam producing means comprising:

l. cathode means emitting the electrons to form said beams, and 2. agrid electrode assembly for controlling the electron beams, said gridelectrode assembly comprising: a. a first grid electrode, b. a secondgrid electrode arranged to act on each of said beams, and c. auxiliaryelectrode means arranged between said first and second grid electrodesin the path of at least one of said beams and having a voltage appliedthereto which is related to the potentials of said first and second gridelectrodes for achieving independent control of the modulatingconditions for said beams; and

C. Electron lens means common to all of said beams and comprising mainfocusing lens means positioned to dispose the optical center thereofsubstantially at said location in the tube, said lens means comprising aplurality of electrodes arranged between said grid electrode assemblyand said screen.

2. A cathode ray tube according to claim 1, in which said auxiliaryelectrode means is provided for each ofsaid beams.

3. A cathode ray tube according to claim 1, in which there are three ofsaid electron beams directed by said beam producing means in a commonplane, said auxiliary electrode means includes an auxiliary electrodefor each of said three beams, and the voltage applied to the auxiliaryelectrode for the central one of said three beams is the same as thepotential of said second grid electrode.

4. A cathode ray tube according to claim 1, in which said plurality ofelectron beams consists of a central beam and two side beams directed bysaid beam producing means in a common plane, and said auxiliaryelectrode means consists of an auxiliary electrode only for each of saidside beams.

5. A cathode ray tube according to claim 4, in which said second gridelectrode has a central projection extending toward said first gridelectrode through which said central beam passes.

6. A cathode ray tube according to claim 4, in which there areindependent cathode means and an independent first grid electrode foreach of said beams, said voltage applied to each auxiliary electrode isdifferent from said potential of said second grid electrode, and saidcathode means and first grid electrode for said central beam arearranged relative to each IOIO45 0457 other to provide said central beamwith a cutofi' voltage that is equal to the cutoff voltage for each ofsaid side beams.

7. A cathode ray tube according to claim 6, in which said voltageapplied to each said auxiliary electrode is greater than said potentialof said second grid electrode.

8. A cathode ray tube according to claim 7, in which said cathode meansfor said central beam is disposed closer to the respective first gridelectrode than are the cathode means and said first grid electrodes forthe side beams.

9. A cathode ray tube according to claim 7, in which said first gridelectrodes have apertures fog the passage of the respective beamstherethrough, and the aperture of said first grid electrode for saidcentral beam is of greater size than the apertures of the first gridelectrodes for the side beams.

10. A cathode ray tube according to claim 7, in which said auxiliaryelectrodes are disposed relatively near to the respective first gridelectrodes.

11. A cathode ray tube according to claim 6, in which said voltageapplied to each said auxiliary electrode is less than said potential ofsaid second grid electrode.

12. A cathode ray tube according to claim 11, in which said auxiliaryelectrodes are disposed approximately midway between the respectivefirst grid electrodes and said second grid electrode.

13. A cathode ray tube according to claim 11, in which said cathodemeans for said central beam is disposed further from the respectivefirst grid electrode than are the cathode means and said first gridelectrodes for the side beams.

'14. A cathode ray tube according to claim ll, in which said first gridelectrodes have apertures for the passage of the respective beamstherethrough, and the aperture of said first grid electrode for saidcentral beam is of smaller size than the apertures of the first gridelectrodes for the side beams.

i III t i

1. A cathode ray tube comprising: A. A phosphor screen; B. Beamproducing means to form a plurality of separately modulatable electronbeams to impinge on said screen after intersecting each other at alocation in the tube between said beam producing means and said screen,said beam producing means comprising:
 1. cathode means emitting theelectrons to form said beams, and
 2. a grid electrode assembly forcontrolling the electron beams, said grid electrode assembly comprising:a. a first grid electrode, b. a second grid electrode arranged to act oneach of said beams, and c. auxiliary electrode means arranged betweensaid first and second grid electrodes in the path of at least one ofsaid beams and having a voltage applied thereto which is related to thepotentials of said first and second grid electrodes for achievingindependent control of the modulating conditions for said beams; and C.Electron lens means common to all of said beams and comprising mainfocusing lens means positioned to dispose the optical center thereofsubstantially at said location in the tube, said lens means comprising aplurality of electrodes arranged between said grid electrode assemblyand said screen.
 2. a grid electrode assembly for controlling theelectron beams, said grid electrode assembly comprising: a. a first gridelectrode, b. a second grid electrode arranged to act on each of saidbeams, and C. auxiliary electrode means arranged between said first andsecond grid electrodes in the path of at least one of said beams andhaving a voltage applied thereto which is related to the potentials ofsaid first and second grid electrodes for achieving independent controlof the modulating conditions for said beams; and C. Electron lens meanscommon to all of said beams and comprising main focusing lens meanspositioned to dispose the optical center thereof substantially at saidlocation in the tube, said lens means comprising a plurality ofelectrodes arranged between said grid electrode assembly and saidscreen.
 2. A cathode ray tube according to claim 1, in which saidauxiliary electrode means is provided for each of said beams.
 3. Acathode ray tube according to claim 1, in which there are three of saidelectron beams directed by said beam producing means in a common plane,said auxiliary electrode means includes an auxiliary electrode for eachof said three beams, and the voltage applied to the auxiliary electrodefor the central one of said three beams is the same as the potential ofsaid second grid electrode.
 4. A cathode ray tube according to claim 1,in which said plurality of electron beams consists of a central beam andtwo side beams directed by said beam producing means in a common plane,and said auxiliary electrode means consists of an auxiliary electrodeonly for each of said side beams.
 5. A cathode ray tube according toclaim 4, in which said second grid electrode has a central projectionextending toward said first grid electrode through which said centralbeam passes.
 6. A cathode ray tube according to claim 4, in which thereare independent cathode means and an independent first grid electrodefor each of said beams, said voltage applied to each auxiliary electrodeis different from said potential of said second grid electrode, and saidcathode means and first grid electrode for said central beam arearranged relative to each other to provide said central beam with acutoff voltage that is equal to the cutoff voltage for each of said sidebeams.
 7. A cathode ray tube according to claim 6, in which said voltageapplied to each said auxiliary electrode is greater than said potentialof said second grid electrode.
 8. A cathode ray tube according to claim7, in which said cathode means for said central beam is disposed closerto the respective first grid electrode than are the cathode means andsaid first grid electrodes for the side beams.
 9. A cathode ray tubeaccording to claim 7, in which said first grid electrodes have aperturesfor the passage of the respective beams therethrough, and the apertureof said first grid electrode for said central beam is of greater sizethan the apertures of the first grid electrodes for the side beams. 10.A cathode ray tube according to claim 7, in which said auxiliaryelectrodes are disposed relatively near to the respective first gridelectrodes.
 11. A cathode ray tube according to claim 6, in which saidvoltage applied to each said auxiliary electrode is less than saidpotential of said second grid electrode.
 12. A cathode ray tubeaccording to claim 11, in which said auxiliary electrodes are disposedapproximately midway between the respective first grid electrodes andsaid second grid electrode.
 13. A cathode ray tube according to claim11, in which said cathode means for said central beam is disposedfurther from the respective first grid electrode than are the cathodemeans and said first grid electrodes for the side beams.
 14. A cathoderay tube according to claim 11, in which said first grid electrodes haveapertures for the passage of the respective beams therethrough, and theaperture of said first grid electrode for said central beam is ofsmaller size than the apertures of the first grid electrodes for theside beams.